1. Field of the Invention
This invention relates to the manufacture of semiconductor device assemblies and, more particularly, to methods of assembly of semiconductor devices using room temperature adhesives and to the assembled semiconductor device and lead frame using room temperature adhesives.
2. State of the Art
To manufacture a semiconductor device assembly, a semiconductor device, typically called a die or chip, is typically attached to a lead frame. A lead frame is a structure having a plurality of leads, bus bars, or other connecting structure to be electrically connected with the semiconductor device. In a conventional lead frame, the lead frame includes a die paddle to which the semiconductor device is attached and a plurality of leads extending inwardly to surround the periphery of a semiconductor device mounted on the die paddle. Subsequently, a plurality of wire bonds are made to connect the bond pads on the active surface of the semiconductor device to the ends of the leads extending around the periphery of the semiconductor device. In a leads-over-chip type lead frame (LOC lead frame), the lead frame is formed having no die paddle and having a plurality of leads which extends over the active surface of the semiconductor device being secured thereto to support the semiconductor device, with a plurality of wire bonds being formed between the bond pads on the active surface of the semiconductor device and the ends of the leads of the lead frame.
A conventional lead frame or LOC lead frame may serve other functions. That is, it may assist in heat dissipation during manufacture, increase the structural strength of the assembled semiconductor device as well as provide convenient locations to make electrical connections.
In order to attach the semiconductor device to the lead frame, different adhesives and adhesively coated tapes have been suggested. For example, U.S. Pat. No. 5,304,582 (Ogawa) shows use of adhesive tape with different adhesives on opposite sides for attaching a die to a lead frame. U.S. Pat. No. 5,548,160 (Corbett et al.) discloses use of adhesives including adhesives that have a core.
Typically, in attaching a LOC lead frame to a semiconductor device, a double-coated adhesive tape is applied between the active surface of the semiconductor device and the lead frame. The adhesive tape is typically an insulating carrier with a polymer adhesive on both sides to mechanically interconnect the lead frame and the semiconductor device. The tape composition and the amount of polymer adhesive used on the tape varies with the size of the semiconductor device. It is desired to use the least amount of adhesively coated tape to attach a semiconductor device to a lead frame to attempt to minimize problems. Too much polymer adhesive added to the tape can cause a coefficient of thermal expansion mismatch between the lead frame, the polymer adhesive and the semiconductor device which can contribute to the failure of the packaged device. Reducing the size of the tape to enhance performance may involve reprocessing the tape at some increased cost and at some difficulty for the smaller sizes.
If an adhesive, as opposed to an adhesively coated tape, is used to attach the semiconductor device to the lead frame, the quantity of adhesive must be carefully controlled in the dispensing process. The bond time for the adhesive is recognized to be difficult to control and can vary greatly with variations in adhesive viscosity, adhesive application temperature and amount of adhesive used. Further, adhesive can bleed from under the lead finger and interfere with the attachment of other lead fingers of the lead frame. Also, use of an adhesive (at elevated temperatures) may also lead to a nonuniform bond line between the semiconductor device and the lead frame. That is, the semiconductor device is not generally in alignment with the lead frame, causing different spacings to be present between the lead fingers and the bond pads on the active surface of the semiconductor device, thereby affecting wire bonding operations. Additionally, uneven application of adhesive or non-uniform adhesive viscosity can lead to tilting of the semiconductor device relative to the lead frame. An uneven or tilted relationship has been determined to be a factor that reduces semiconductor device assembly quality and leads to failures.
Therefore, an adhesive with better qualities suitable for direct bonding of the semiconductor device to the lead fingers of a lead frame is desirable.
A semiconductor device assembly is formed by joining a lead frame and a semiconductor device. The lead frame includes a plurality of lead fingers, each lead finger having a lower attaching surface for adhesive attachment to portions of the active surface of a semiconductor device.
A non-conductive polymer adhesive is selected from the group of adhesives that is tacky and compliant at room temperature, easily applied to a substrate, such as through the use of a stencil, and easily cured to a predetermined degree. The non-conductive polymer is applied at room temperature, either to the lower surface of the lead fingers of the lead frame or to portions of the active surface of the semiconductor device for compression therebetween. The lead fingers are connected by wire bonds to the bond pads on the active surface of the semiconductor device.
The adhesive preferably has a first copolymer material selected from the group of copolymers that includes isobutyl compounds and a second material that is from a group of metal oxides that includes titanium dioxide. Preferably, the first material is isobutyl acetal diphenol copolymer. More preferably, the second material is titanium dioxide.
In a preferred composition, the adhesive has about 75 percent to about 95 percent of isobutyl acetal diphenol copolymer and, respectively, from about 25 percent to about 5 percent of titanium dioxide.
Methods of assembling the semiconductor device assembly include providing a lead frame and a semiconductor device. A non-conductive polymer adhesive is selected from the group that is tacky and compliant at room temperature and is applicable to a substrate through a stencil. The adhesive is applied to one of the semiconductor device and a surface of the lead frame at room temperature. The lead frame and the semiconductor device are urged together at room temperature to attach the semiconductor device to the lead frame and with electrical connections between the lead fingers of the lead frame and the bond pads on the active surface of the semiconductor device made by wire bonds extending therebetween.
In the preferred methods of assembly, the adhesive may be from the groups of adhesives as described hereinbefore.
In an alternate configuration, a stencil is provided with the adhesive, at room temperature, applied to either a surface of each lead finger of the lead frame or portions of the active surface of the semiconductor device. The lead frame and the semiconductor device are thereafter positioned relative to each other and urged together to effect attachment and electrical connection, if desired.